A fuel cell is a power generation system to generate electrical power from chemical energy. This is classified in several types based on the operation mechanisms and materials to be used, such as alkaline-type fuel cell, phosphoric-acid fuel cell, molten carbonate fuel cell, solid-electrolyte fuel cell and solid-polymer type fuel cell, and various forms of fuel cells have been proposed and examined. Among these, the alkaline fuel cell and solid-polymer type fuel cell are expected to use as medium to small-size low-temperature operating-type fuel cells for stationary power source and in-vehicle use as well as portable power source since the operating temperature is as low as 200° C. or less.
The solid-polymer type fuel cell uses solid polymer such as ion-exchange resin as its electrolyte, and has relatively low operation temperature. The solid-polymer type fuel cell has, as shown in FIG. 1, a basic structure wherein a space surrounded by cell bulkhead 1 having a fuel flow hole 2 and oxidizing agent gas flow hole 3, respectively communicated with outside, is divided by a membrane assembly in which an anode 4 and a cathode 5 are respectively bonded to both surfaces of a solid polymer electrolyte membrane 6 to form an anode chamber 7 communicated with outside via the fuel flow hole 2 and a cathode chamber 8 communicated with outside via the oxidizing agent gas flow hole 3. Then, in the solid polymer type fuel cell having the above basic structure, hydrogen gas or liquid fuel such as methanol, etc. is supplied into the anode chamber 7 via the fuel flow hole 2, and oxygen or oxygen containing gas such as air to act as an oxidizing agent is supplied into the cathode chamber 8 via the oxidizing agent gas flow hole 3. Furthermore, an external load circuit is connected between both electrodes to generate electrical energy by the following mechanism.
When using a cation-exchange membrane as the solid polymer electrolyte membrane 6, a proton (hydrogen ion) generated by contacting a fuel with a catalyst included in the anode 4 conducts in the solid polymer electrolyte membrane 6 and moves into the cathode chamber 8 to generate water by reacting with oxygen in the oxidizing agent gas in the cathode 5. On the other hand, an electron, generated in the anode 4 simultaneously with the proton, moves to the cathode 5 through the external load circuit, so that it is possible to use the energy from the above reaction as an electrical energy.
In the solid-polymer type fuel cell wherein the cation-exchange membrane is used for a solid electrolyte membrane, only an expensive noble metal catalyst is usable as the catalyst in the electrode due to the strongly acidic reaction field.
Then, it has been examined to use an anion-exchange membrane instead of the cation-exchange membrane. In a fuel cell using the anion-exchange membrane, a catalyst other than noble metal can be used because the reaction field is basic. However, in this case, a mechanism for generating electrical energy in the solid-polymer type fuel cell is different in ion species moving in the solid polymer electrolyte membrane 6 as below. Namely, hydrogen or methanol, etc. is supplied into the anode chamber, and oxygen and water are supplied into the cathode chamber, so that the catalyst included in the electrode is contacted with the oxygen and water in the cathode 5 to generate hydroxy-ion. This hydroxy-ion conducts in the solid polymer electrolyte membrane 6 formed by the above anion-exchange membrane to move into the anode chamber 7 and reacts with the fuel in the anode 4 to generate water. An electron generated as a result of the reaction in the anode 4 moves to the cathode 5 through an external load circuit, so that the reaction energy is used as an electrical energy.
The above fuel cell with a mechanism in which hydroxy-ion moves in the membrane is called as an alkaline fuel cell. Therefore, the solid-polymer type fuel cell using an anion-exchange membrane as a solid-polymer type fuel cell electrolyte membrane can also be classified into alkaline fuel cells.
In the alkaline fuel cells, atmospheres of both electrodes are basic, and choices for available catalyst types are increased, which results in the following advantages. For example, overvoltage of oxygen reduction can be reduced, and furthermore, it is expected as well to improve voltage by selecting a cathode catalyst inactive to the fuel passing through the membrane.
In an example of the alkaline fuel cells using anion-exchange membrane, hydrogen is supplied to the anode side, and oxygen or air is supplied to the cathode side to generate electricity (Patent Article 1 & Nonpatent Literature 1).    [Patent Article 1] Japanese Unexamined Patent Publication No. 2007-042617    [Nonpatent Literature 1] Journal of Power Sources 2008, vol. 178, p. 620
In an alkaline fuel cell using an electrolyte membrane which is an anion-exchange membrane, hydrogen gas is advantageous as its fuel gas because it is easy to obtain high output when a highly-diffusible and highly-active catalyst is usable. However, while the alkaline fuel cell using an anion-exchange membrane as an electrolyte membrane has various advantages over those using a cation-exchange membrane as mentioned above, its actual output is not as high as expected even when using the hydrogen gas.
The purpose of the present invention is to develop a method to obtain higher output than before in alkaline fuel cells using an electrolyte membrane which is an anion-exchange membrane.